Vehicular operation of portable cellular telephones in remote areas is often desired, such that there is a need for higher transmit power capability in portable phones used in vehicles. Previously, power control circuitry (for generating a second set of higher output power levels) has been provided within the main body of the portable telephone. For example, in U.S. Pat. No. 4,636,741 certain of the configurations described include the capability of generating a second set of output power levels within the main unit. These configurations, however, generally result in an increase in the size and weight of the portable unit, whereas in today's cellular industry it is desirable to reduce the size and weight of portable phones.
Since a battery with a voltage higher than is typically provided by portable batteries is required to generate a second set of higher output power levels, most manufacturers now provide the power amplifying capability in a separate unit (i.e., booster) inside the vehicle so that the vehicle's battery can be utilized. This, of course, reduces the size, weight and cost of the portable phone. The microcomputer in the portable phone is used to control the output from the power booster, typically through data communication with a second microcomputer in the booster using a bi-directional serial line--see U.S. Pat. No. 4,636,741 (col. 6, lines 15-27). In such an arrangement, the second microcomputer, in response to the main unit, controls the output power of the booster by setting an eight-step attenuator or a digital to analog converter.
Alternately, the same result has been accomplished by using a data bus so that the microcomputer in the portable phone can directly control an eight-step attenuator in the power booster--see U.S. Pat. No. 4,636,741 (col. 5, line 38 to col. 6, line 14).
Although the existing designs substantially solve the problems of size and weight of portable phones, the utilization of a second microcomputer, a serial data line and/or a data bus, along with associated software for generating a second set of output power levels, makes the design/operation of the known devices complex and the cost quite high.